ALBERT

Description: Seismic hazard studies provide indicators of seismic motion that are expressed for “free-field,” that is, representative of the ground motion exactly at the free surface, without disturbances due to interactions between soil and buildings or other structures. Most of these studies are based on ground-motion prediction equations, which are, themselves, formulated to predict free-field motion, as they are derived from similarly free data. However, is this really the case? In this study, we use several examples to illustrate how small structures hosting permanent strong-motion stations (often anchored on small concrete slabs) generate soil–structure interaction effects that can amplify the high-frequency part of the earthquake signal (〉10  Hz) by up to a factor of 2–3 for stations on soils. We also show that the installation depth of a station, even if very shallow (i.e., a few meters), can change the recorded response, mainly by deamplifying the signal in high frequencies (〉10  Hz) by a factor up to 0.3. Such effects imply that there are actual differences between recorded and true free-field signals. Depending on the housing conditions, these effects can have significant impact on response spectra at high frequencies, and on measurements of the κ parameter. It is, thus, becoming clear that such effects should be taken into account in studies involving high-frequency seismic motion. To do so, scientists need a detailed description of the conditions of installation and housing of seismological and accelerometric stations, which often lacks from the metadata distributed through the various, commonly used web services. Increasing such information and facilitating the access to it would allow the identification of stations that are problematic and of those that are truly close to free-field recording conditions. In a subsequent step, it would be important to quantify the modification curve of the response of stations that experience such effects.

Description: This work presents the implementation of a high standards dataset of earthquake ground motions recorded in Metropolitan France between 1996 and 2016. This dataset is intended to serve the scopes of a large seismological community; however, its primary aims are engineering seismology and earthquake engineering applications. The dataset includes records from broadband and accelerometric sensors provided by permanent and temporary seismic networks operated by French research institutions and partners grouped within the Réseau Sismologique et géodésique Français (RESIF) consortium (see Data and Resources). All the waveforms are first homogenized to acceleration units and underwent a full quality control process including: (1) visual verification and manual phase picking, (2) processing scheme, and (3) consistency check by residual analysis. These make the dataset fully compliant with international standards. Finally, the RESIF dataset includes more than 6500 quality-checked records from 468 earthquakes recorded at 379 stations. Over the 379 stations included in the dataset, 177 are permanent stations (RA and FR networks) and 202 are temporary stations (YP and X7 networks). The dataset is complete down to magnitude ML 3.5 over the whole French territory and less (about ML 2.5) in the eastern part. Magnitudes range from ML 2.4 to 5.6 and from Mw 2.0 to 5.2, whereas distances range from less than 1 to 600 km. The disseminated dataset is constituted of (1) a versioned flatfile containing the metadata and main intensity measures computed on each processed record, (2) time histories, and (3) response spectra for several damping values and Fourier spectra.

Description: SUMMARY To evaluate the site response using both empirical approaches (e.g. standard spectral ratio, ground motion models (GMMs), generalized inversion techniques, etc.) and numerical 1-D/2-D analyses, the definition of the reference motion, that is the ground motion recorded at stations unaffected by site-effects due to topographic, stratigraphic or basin effects, is needed. The main objective of this work is to define a robust strategy to identify the seismic stations that can be considered as reference rock sites, using six proxies for the site response: three proxies are related to the analysis of geophysical and seismological data (the repeatable site term from the residual analysis, the resonance frequencies from horizontal-to-vertical spectral ratios on noise or earthquake signals, the average shear wave velocity in the first 30 m); the remaining ones concern geomorphological and installation features (outcropping rocks or stiff soils, flat topography and absence of interaction with structures). We introduce a weighting scheme to take into account the availability and the quality of the site information, as well as the fulfillment of the criterion associated to each proxy. We also introduce a hierarchical index, to take into account the relevance of the proposed proxies in the description of the site effects, and an acceptance threshold for reference rock sites identification. The procedure is applied on a very large data set, composed by accelerometric and velocimetric waveforms, recorded in Central Italy in the period 2008–2018. This data set is composed by more than 30 000 waveforms relative to 450 earthquakes in the magnitude range 3.2–6.5 and recorded by more than 450 stations. A total of 36 out of 133 candidate stations are identified as reference sites: the majority of them are installed on rock with flat topography, but this condition is not sufficient to guarantee the absence of amplifications, especially at high frequencies. Seismological analyses are necessary to exclude stations affected by resonances. We test the impact of using these sites by calibrating a GMMs. The results show that for reference rock sites the median predictions are reduced down to about 45 per cent at short periods in comparison to the generic rock motions.